Still, research suggests it will continue growing as a political force. Four major elections from the past year show how this dynamic can play out in different ways.
A Gradual Gain, Only Now Obvious
Marine Le Pen, the leader of the far-right National Front party in France, demonstrated in the presidential election’s first round how far her movement has come. In the second round, if polls prove correct, she will demonstrate how far it remains from taking power.
Over time, the party has steadily improved its performance. In 1988, it won 14 percent. In 2002, the only other year it made it to the second round, 17 percent; in 2012, 18 percent. This year, Ms. Le Pen won 21 percent of the vote, which put her in the second round.
This has been the story of populism across Europe: a steady rise over the course of decades.
Since the 1960s, populist parties have doubled their average share in European elections and tripled their share of seats in European legislatures, according to a recent paper by the political scientists Ronald Ingleheart and Pippa Norris.
Today, those parties hold 13 percent of parliamentary seats and win roughly the same share of national votes, they found. Ms. Le Pen is outperforming the average — every country is a bit different, after all — but not by much.
This process has been so gradual as to have been barely noticed until now. It only feels sudden.
While elections are unpredictable and anything is possible, Ms. Le Pen is projected to lose the second round by as much as 20 percent. So though the populist wave is rising, its pace is too slow to alone propel her into power.
This is the nature of populism’s awkward size. It is too small to reliably win national elections. But it is large enough to reframe politics, in France and elsewhere, as a debate between globalism and ethno-nationalism. Trend lines continue to point upward.
When the System Makes the Difference
For much of the 2016 American presidential contest, it looked as if Donald J. Trump would take a path similar to Ms. Le Pen’s: unprecedented victory in the Republican primary followed by humiliating defeat in the general election.
In competitive primary contests, Mr. Trump’s support hovered around 30 percent. Polls showed him to be wildly unpopular among supporters of more mainstream candidates, as well as the majority of Democrats.
This led many analysts to conclude that, nationally, Mr. Trump’s support had a ceiling of perhaps 40 percent, again on par with Ms. Le Pen.
But there was one crucial difference: decades of partisan polarization between America’s two-party system.
While Mr. Trump did win some working-class Democratic voters, his greatest leap came when once-skeptical Republicans rallied around him, pushing him past his supposed ceiling. And the Electoral College system meant Mr. Trump could win even while losing the popular vote by two percentage points.
Whereas France’s center-right parties feel comfortable rejecting Ms. Le Pen, the Republican Party saw little choice but to back Mr. Trump. And while Republican voters may have been skeptical of Mr. Trump, polarization made their side’s victory — and the other’s defeat — feel imperative.
Polarization, though less extreme elsewhere, is playing out across Western societies, driving once-centrist voters toward more extreme parties on the right and left — spaces often occupied by populists.
Too Small to Win, Too Large to Ignore
Western establishments, after months of alarm, celebrated the Dutch elections in March as a sign that the populist tide had been turned back.
The far-right Party for Freedom, led by Geert Wilders, grew to 20 seats from 15 to become his country’s second-largest party, tracking populism’s continental rise.
In the end, though, it won only 13 percent of the vote, slightly less than polls had projected and far too little to force its way into a governing coalition. Dutch voters seemed to have ended the string of populist victories.
But Mr. Wilders had been held back by the mathematical tyranny of parliamentary systems, not just by an anti-populist backlash.
In parliamentary systems, votes tend to be split across several parties, with none securing a majority on its own. To govern, a party has to form a majority coalition with other parties.
This means that as long as a populist party does not win more than 50 percent of the vote — virtually impossible in systems like that of the Netherlands — the other parties can band together to form a coalition excluding it, known as a “cordon sanitaire.”
Yet even though Mr. Wilders did not take control of government, his movement and policies advanced.
The center-right party, which leads the government, held power in part by co-opting Mr. Wilders’s message, particularly on immigration. Mark Rutte, the prime minister, told migrants in an open letter shortly before the vote, “Act normal or leave.”
Similar forces could play out in French legislative elections in June. Even if Ms. Le Pen loses, establishment parties have fractured and center-right parties have grown more populist. Her centrist opponent, Emmanuel Macron, will face pressure to move right as well.
Over time, these dynamics could further accelerate populism’s rise, Cas Mudde, a Dutch political scientist, wrote in Foreign Affairs in September.
As more populist parties become their country’s second or third largest, mainstream parties will have to form more “cordons sanitaire” to keep them out. For populist voters, this feels like an establishment conspiracy to repress popular will, deepening outrage at a seemingly unresponsive system.
Even if populist parties are often too small to take power, when the correct forces align, they are powerful enough to reshape politics.
This is how the U.K. Independence Party, or UKIP, helped bring about the British exit from the European Union.
After years of single-digit shares in national elections, the party won 13 percent of votes in 2015, again tracking with the measures of broader trends.
It won only one seat in Parliament, electing a representative who later quit the party. But that victory helped cause the center-right Conservative Party to fear that it could lose power, or that its centrist leadership could fall, unless it co-opted UKIP’s appeal. So the party brought a referendum on leaving the European Union. It seemed to believe the referendum would fail. After all, 13 percent wasn’t much.
But so-called Brexit passed, with 52 percent, because of support from voters whom the scholars Jonathan Mellon and Geoffrey Evans termed “UKIP curious,” those who were unwilling to consistently support the party but were receptive to its message.
“We live in an unprecedented era of volatile party support,” the scholars wrote, meaning that even a party as marginal as UKIP could put enough pressure on the system to force its policy onto the agenda — and that enough latent support existed to secure a majority, even if for just one election on one day.
This hardly means that UKIP will someday take power, or even necessarily win a second seat in Parliament. But it hardly needs to. As Brexit proves, the populist wave can do plenty at 13 percent.